Radiation dosimeters, which measure the dose of incident radiation to which people, animals, plants or inanimate objects are exposed, are known. Dosimeters have been used in prior art for personal and environmental monitoring, for medical research and for radiation therapy. Of particular interest are small dosimeters, which are worn as personal badges and which need to be reliable and accurate, especially for measurements of low radiation doses. Some of these dosimeters are based on disposable photographic films. Others are non-disposable, such as those based on optically stimulated luminescence. Thermoluminescent dosimeters are among the most important non-disposable personal dosimeters, and may be used for example for in vivo and environmental dosimetry.
Thermoluminescence (TL) is a physical phenomenon exhibited in materials which are irradiated with energetic radiation and subsequently stimulated, using heat, to produce luminescent emission. When thermoluminescent materials are exposed to a flux of electromagnetic radiation such as gamma rays, X-rays or UV rays, to charged particles such as beta rays, to uncharged particles such as neutrons, or to other forms of radiation, electrons within the material are excited from low energy levels to relatively stable traps at higher energy levels. The electrons may stay at these higher energy levels for a long period of time. If the material is heated, the added energy releases the trapped electrons, causing them to fall back to the lower energy levels. This fall is accompanied by the emission of a luminescent emission, i.e. thermoluminescence.
When a thermoluminescent element is heated from some low temperature T0 to some high temperature (e.g. 400° C.) the intensity of the luminescence increases at first (when more electrons in the traps are released) and then decreases (when the number of trapped electrons decreases). This gives rise to a peak in the luminescence, which appears at a certain temperature. If there are several types of traps, several peaks are observed at different temperatures. The graph of the luminescence intensity as a function of temperature is called a “glow curve”. The heights of the peaks (or the integrated area under the glow curve) are found to depend on the radiation exposure dose. In a simple case, the dependence may be linear, which allows the dose to be obtained from a measurement of the glow curve, after a proper calibration. This is the principle of thermoluminescence dosimetry (TLD).
One application of TLD is to monitor radiation exposures of personnel such as medical personnel exposed to X-rays. Each person is required to carry a dosimeter called a “TLD badge.” The badge may comprise more than one TLD element. The badge is assumed to receive the same dose as the carrying person. Periodically, the badges are processed to obtain an exposure record for each person being monitored.
If glow curve measurements are carried out on TLD elements, reliable results are obtained only when each of these elements is heated in a controllable and reproducible fashion (i.e. using the same initial and final temperatures and the same variation of temperature with time). The most desirable way is to heat the elements linearly, so that for each element the temperature follows the formula Tel=T0+αt, where T0 is the initial temperature, Tel is the temperature of the sample at time t, and α is the beating rate. It is advantageous to heat the samples as fast as possible, since in this case the glow peaks are sharp and easy to measure, and more samples can be measured in a given time period.
Different materials have been used for TL dosimetry (and for making TLD elements). These include Na2SO4, MgSO4, Y2O3, Al2O3, CaF2, SrF2 and BaF2, doped materials, such as CaSO4:Tm, CaF2:Mn, Al2O3:C, LiF:Cu, Mg, Pr, as well as many other materials. The TLD elements are normally made from single crystals, from pressed powders, from thin layers deposited on substrates, from small particles embedded in glass or in polymers, etc. In some cases the elements are exposed (uncased) during the TL measurement. In other cases each element is placed within a tiny plastic bag, which is part of the personal badge, so that the heating and the luminescence measurements can be carried out without removing the element from the plastic bag. The TLD elements may have different geometrical shapes, e.g. plates, discs, rods, pellets, fibers, etc.
Several methods have been used in the past for heating the TLD elements:
(a) Contact Heating—the element is heated by a tiny heater placed in close contact and whose temperature is controlled. The heating in this case is highly non-uniform and relatively slow. Also, the results are non-reproducible, because the temperature of the element depends on the contact between the element and the heater.
(b) Hot Gas Heating—the element is heated by a stream of hot gas, whose temperature its controlled. This method provides more uniform and faster heating, but the whole system is much more complicated and expensive.
(c) Heating by Incandescent Lamps—in this case the heating depends on the absorption of radiation emitted from an incandescent source in the element or in the substrate on which the element is placed. The reproducibility of this method is insufficient, and (especially with substrate heating) the heating rate is also insufficient.
(d) Laser Heating—in some cases the laser radiation may be directly absorbed by the TLD element. In other cases, the laser radiation is absorbed in a matrix (e.g. glass or polymer) embedding small particles of the TL material. The laser heats the matrix, which in turn heats the particles. In most cases the laser power is monitored and it is assumed that a given power generates a predetermined temperature increase. The main problem with this method is that the laser power changes with time and its distribution is non-uniform. This may lead to distorted glow cures and to inaccurate results.
(e) Rapid Heating by Light—in this case an energetic pulse from a laser (or from an incandescent light source) rapidly heats the TL element. The intensity of the emitted luminescence is very high and easy to measure. The main disadvantage of this method is in the very limited control of the heating process, which leads to non-reproducible glow curves.
(f) RF Heating: TLD elements are bonded to graphite plates and heated by induction, using a radio frequency (RF) generator. As in (e), the heating control is difficult and the resulting glow curves may be non-reproducible.
Exemplary methods and systems may be found in a number of prior art publications, for example in U.S. Pat. Nos. 3,531,641, 3,729,630, 3,975,637, 4,204,119, 4,638,163, 4,835,388, 4,839,518, 5,041,734, 5,081,363, 5,606,163, 6,005,231 and 6,414,324. None of these methods and systems measures the temperature of the TLD element in real time to enable accurate control of the TLD heating rate. For example, U.S. Pat. No. 6,005,231 discloses a method and apparatus for measuring radiation doses based upon thermoluminescence. A heat energy sensor is provided for the beat source for detecting the heat energy output from the heat source toward the element. The temperature of the element is calculated on the basis of the detected heat energy. The calculated temperature is used to determine if remedial action is necessary. For instance, the calculated temperature may be compared with a predetermined optimum heating temperature. If the calculated temperature deviates from the predetermined optimum heating temperature, responsive action is taken. A temperature increase rate may also be calculated. The calculated increase rate would be compared with a predetermined heating rate. The heating device would increase its heat energy output if the calculated increase rate is lower than the predetermined heating rate and decrease the heat energy if the calculated increase rate is higher than the predetermined heating rate. In summary, the inventors do not measure directly the temperature of the TLD element, and in fact state that it is almost impossible to measure the temperature of the TLD element during heating.
All mentioned prior art methods assume that the temperature Tel of the TLD element itself should increase reproducibly during the heating phase. Theoretically, the most convenient heating scheme is that in which the temperature increases linearly with time. However, Tel is not directly controlled in any of these methods. In some cases, a thermocouple may be placed in contact with the TLD element to provide a Tel measurement. However, since the physical contact between thermocouple and TLD element may vary between elements and may change in time for the same element, such temperature measurements are inaccurate. This remains a common problem in all TLD systems. Consequently, even if the heating source behaves reproducibly, the real heating rate of the TLD elements may not be reproducible.
The temperature Tel of the TLD element can be determined by measuring the thermal infrared radiation emitted from its surface. The intensity I of the radiation emitted from a surface area A is given by the expression I=AεσT4, where ε is the emissivity of the element and σ is the Stephan-Boltzmann constant. The spectral distribution of thermal radiation is derived from Planck's black body theory. The dependence of the wavelength λmax at which a black body emits at maximum intensity on its temperature is known as Wien's displacement law: λmax T=2898 μmK. Therefore, most of the thermal radiation of a body near room temperature (T≈300° K) is in the middle infrared (mid-IR) in the spectral range 3-30 μm. This radiation can be easily measured by infrared detectors, which may exemplarily be thermal detectors, such as pyroelectric, thermoresistive and MEMS devices, many of whom operate at room temperature, or photonic (i.e. quantum) detectors such as HgCdTe, many of whom are cooled by liquid nitrogen or thermoelectrically. This method of infrared radiometry therefore serves for infrared thermometry.
The mid-IR radiation emitted from the surface of the TLD element can be collected and focused on the infrared detector using standard optical elements, such as mirrors or lenses. The emitted mid-IR radiation can also be carried through infrared transmitting optical fibers, only a few of which are transparent in the mid-IR range. Optical fibers made of silver halides are among the best candidates for that purpose. They are highly transparent in the mid-infrared, in the spectral range 3-30 μm, with losses of about 0.2 dB/m at 10.6 μm.
IR temperature measurements based on detection of IR radiation emitted by a heated body are known, see e.g. Remote IR sensing of temperature, including through the use of fibers that conduct the IR radiation to a detector, is also known, see e.g. S. Sade, O. Eyal, V. Scharf and A. Katzir, “Fiberoptic Infrared Radiometer for Accurate Temperature Measurements,” Applied Optics, Vol. 41, no. 10, pp. 1908-1914 (2002). However, the use of such measurements for determining the temperature of a TLD element in real-time, and the use of this data (temperature readings) in close loop control of the heating of the element is unknown.
In conclusion, all prior art methods are disadvantageous in that the temperature of the TLD elements is not well monitored and controlled and the heating of such elements is generally not well controlled. Consequently, glow curves suffer from irreproducibility and so do dosimetry results. There is therefore a widely recognized need for, and it would be highly advantageous to have a TLD measurement system and method in which the temperature and heating rate of each TLD element is known and controllable in real-time.